The present invention relates to circuits for controlling a contactor coupled to loads that may hold a permanent charge, and more particularly to a circuit for safely connecting and disconnecting the load.
In power systems, a main contactor is often used to connect and disconnect loads that hold a permanent charge, for example battery packs or super capacitors, from the power bus. Deenergizing the power bus eliminates the risk of accidents, including physical injury, when the system is left unsupervised or when the system needs to be maintained, repaired, assembled, or dissembled. Ideally, the contactor is often located as close to the power source as possible to minimize the portion of the system that remains xe2x80x9clivexe2x80x9d.
When connecting a power source to an uncharged capacitive load, the contactors are subjected to large inrush currents when the contactor first closes. This temporary current surge can be problematic as the size of the load controlled by a single contactor increases. Though the inrush may be short-lived, this level of surge can wreak havoc on the contacts of even a relatively large relay having a high current rating.
Existing power systems address the problem described above by using a single heavy-duty relay having large contacts. But contactors of this type tend to be both costly and bulky in size.
Another approach is to use two relatively small relays connected in parallel, with one having a current-limiting resistive element in series therewith. Such a switching circuit is shown in FIG. 1. In operation, relay RL1 is closed for a short time while relay RL2 remains open. As relay RL1 closes, current from the power source rushes through the resistor R to charge up the capacitive load. The opening and closing of the contactors in these systems is based on time or mechanical design. Typically, after a predetermined time, RL2 is closed.
Other solutions, as shown in FIG. 2, use a hybrid switching circuit, which combines a relay of the type having two sets of contacts, and a semiconductor switch, such as a triac. This circuit operates as follows: When an input signal is applied to the relay, the contacts A close first, thereby causing current to immediately flow through resistor R to the gate lead of triac Q. Upon triggering the triac, current flows from the power source to the load, through the triac. After a predetermined time period, the B contacts close, allowing load current to flow unimpeded from the source to the load. At this point, both sets of relay contacts are closed. When the input signal is removed, the B contacts open first, thereby causing load current to again flow in the triac. Subsequently, when the contacts A open, the load current becomes zero and is cut off by the triac.
Another switching circuit, as shown in FIG. 3, comprises a pair of relays RL3 and RL4, preferably connected in parallel, with one of such relays having a controllably conductive device, such as an electronic switch, and preferably a triac Q1, connected in series therewith. With the relays RL3 and RL4 open, an air gap isolates the power source and the load. In closing the relays in sequence, relay RL3 provides a conductive path from the power source to the triac Q1. After a delay, the triac is triggered to provide a conductive path from the power source to the load, and a large current surge (as much as 300 amps) flows to the load, for example an electronic fluorescent ballast with a capacitive front end. After a predetermined period of time, the other relay RL4 is closed to provide a direct conductive path between the power source and ballasts. Subsequently, RL3 may be reopened.
In power systems that do not have a main contactor, the load itself typically incorporates the contactor for connecting and disconnecting the load and controlling inrush currents.
Thus, existing circuits often have a controller circuit or mechanical design that relies on a fixed or variable time period to open or close the contactors. However, the fixed or variable time period may not correspond to the appropriate time to open or close the contactors.
Another problem associated with contactors coupled to loads is that, should the contactor open during operation, the system must be shut down quickly in order to avoid under or over voltages at the loads as well as arching at the contactor. In order to safely shut down the system there needs to be a circuit that can detect if the contactor has unintentionally opened.
Another problem associated with contactors coupled to capacitive loads is how to safely discharge the load so maintenance personnel can safely maintain, repair, assemble, or dissemble the system. In order to safely discharge the load, a path needs to be created in order to bleed away the stored charge.
It would be desirable to provide a contactor control circuit that can overcome the above-mentioned shortcomings.
In view of the foregoing discussion, an object of this invention is to provide a contactor control circuit that is capable i) of pre-charging the loads to control the inrush of current, ii) discharging the load to eliminate the risk of shock due to capacitive charge, and iii) shutting the system down in the event the contactor unintentionally opens during operation.
It is a further object of the invention to provide a circuit for coupling a power source to a load. The circuit comprising a main contactor coupled in series with the power source and the load and a detector circuit coupled to a conductive path in parallel with the main contactor, the detector circuit configured to detect the flow of current through the conductive path.
It is a further object of the invention to provide a circuit for coupling a power source to a load. The circuit comprising a main contactor coupled in series with the power source and the load, the main contactor operable in a conductive state and a nonconductive state, a pre-charge circuit coupled in parallel with the main contactor, the precharge circuit outputting a signal indicative of the current passing through the pre-charge circuit, and a controller circuit for controlling the state of the main contactor based on the signal from the pre-charge circuit.
It is a further object of the invention to provide a circuit for coupling a power source to a load. The circuit comprising a main contactor coupled in series with the power source and the load, a pre-charge circuit coupled in parallel with the main contactor, the pre-charge circuit comprising a current limiting element and a controller circuit for controlling the opening of the main contactor based on a voltage drop across the current limiting element.
It is a further object of the invention to provide a circuit for coupling a power source to a load. The circuit comprising a main contactor coupled in series with the power source and the load, a pre-charge circuit comprising a first conductive path coupled in parallel with the main contactor, a discharge circuit comprising a second conductive path coupled in parallel with the load, and a controller circuit for controlling the opening of the main contactor based on a current through the first conductive path.
It is a further object of the invention to provide a system comprising a power source, a load, a main contactor coupled in series with the power source and the load and a detector circuit coupled to a conductive path in parallel with the main contactor, the detector circuit configured to detect the flow of current through the conductive path.